Abstract: A digital transmission system using standard FM broadcast stations employs an SCA subcarrier which is amplitude modulated with a second subcarrier modulated to have different phase orientations representative of digital data. Bits are separated from a digital data stream in groups and are differentially encoded into one of at least four phase orientations of the second subcarrier. The amplitude modulated SCA subcarrier is combined with the stereo signal output of a stereo multiplexer to form the modulating signal for the station FM modulator. The receiver includes a differential phase detector for reconstruction of the differentially encoded digital data.A system which additionally includes means for modulating the SCA subcarrier with a composite signal having an audio signal in addition to the differentially encoded second subcarrier is disclosed.

Abstract: A 14.4 kilobit/second modem uses an encoding scheme in which groups of five bits are encoded as one of thirty-two (2.sup.5) possible code groups. This is done by using quadrature amplitude modulation and a 6 by 6 space-state constellation which allows a maximum of thirty-six different points to be encoded. Since only thirty-two points are needed the four outer corner points of the constellation are not used. In order to achieve the desired 14.4 KBPS data rate the baud clock must run at 2880 Hz. However, this bandwidth is very close to the maximum bandwidth available on voice-grade telephone lines. Accordingly, data encoding and data recovery techniques must be used which maximize the probabilities of correctly receiving the encoded data signals.

Abstract: In a system in which a central modem individually polls a number of remotely located modems for data, an adaptive equalizer in the central modem is initially trained to minimize communication link interference from a remote modem and develops a set of equalizer coefficients based on the initial training which is transmitted to the remote modem and stored there for future use while the central modem goes on to do the same with other remote modems. When a remote modem is polled, the stored equalizer coefficients are used to set the adaptive equalizer of the remote modem so the adaptive equalizer of the central modem does not have to be retrained for each individual remote modem.

Abstract: A two-wire, carrier-type communication system is provided which enables simultaneous two-way transmission over two-wire circuits in the same spectrum. Carrier signals in quadrature are used to allow separation of signals within the same frequency spectrum. The carrier recovered at each subscriber station is used to demodulate the received signal and, after being phase shifted 90.degree., is applied as carrier to the transmitter modulator. The transmitters at the subscriber stations are thus the same as that at the central office but are locked to the receiver and 90.degree. out of phase. A bidirectional control phase adjustment network at the central station adjusts the phase of the signal received thereby until the received carrier is 90.degree. out of phase with the transmit carrier.

Abstract: A pair of full duplex modems (12, 14), each at one end of a two-wire voice grade telephone channel (16), simultaneously transmit synchronous data at high speeds in both directions. In each modem, the simultaneous transmission and reception are accomplished by frequency division. The high speed operation is obtained by odd/even bit stream division and full response binary to ternary level coding coupled with nine level quadrature amplitude modulation and correction framing techniques.

Abstract: A partial response QAM Modem is provided wherein a transmit signal is generated by modulating band limited, multilevel, partial response precoded data with inphase and quadrature carriers in inphase and quadrature phase channels, respectively, and summing the modulated outputs. The received signal is processed by a passband automatic complex equalizer. The equalized signal is delayed and then demodulated with recovered inphase and quadrature carriers to recover the inphase and quadrature phase baseband signals. The recovered baseband signals are sampled at appropriate instants to produce estimates of the transmitted symbols. These symbols are partial response decoded and processed by error correctors to improve the overall system performance. The error corrector outputs are decoded to convert the multilevel estimated data to binary data and the binary data of the two channels is summed to obtain estimates of the binary data transmitted.

Abstract: A multi-phase, multi-amplitude level modulation method, and modulator, is provided which produces a three-level, ten-phase modulated carrier. The carrier is modulated in three amplitude levels, viz., off, intermediate and high and in ten phase positions, to produce sixteen signal states, one state corresponding to the carrier being off, five intermediate amplitude level states having five equally spaced phase positions, and ten high amplitude level states having ten equally spaced phase positions. A number of different four digit binary coding schemes for the sixteen signal states are disclosed together with other different coding and modulation techniques.

Abstract: A multi-level digital filter system applicable to modems is provided. The incoming data is read serially into M shift registers and read out in parallel by M further shift registers under the control of an M-bit clock, where M=log.sub.2 N and N is the number of levels. After appropriate conditioning by a logic control circuit, the parallel outputs are filtered separately by M digital shaping filters. The shaping filters each comprise a chain of shift registers the outputs of which are weighted by a resistor network and summed to produce a desired time response, which in an exemplary embodiment is the inverse Fourier transform of an ideal low-pass filter. Summing of the filter outputs produces an N-level signal. Specific examples of three-, four-, five-, six- and eight-level systems are disclosed.